1. Field of the Invention
This invention generally relates to a compression bonded type semiconductor device for use in power conversion devices including, but not limited to, gate commutated turn-off (GCT) thyristors.
2. Description of the Background
Gate turn-off (GTO) thyristors have been widely used in large-capacity power electronics. However, background GTO thyristors have the following problem. First, snubber circuitry is required, and second, it is difficult to suppress an increase in snubber loss which occurs with an increase in operation voltages thereof. Fortunately, a specific thyristor device, known as a gate commutated turn-off (GCT) thyristor (which is designed to eliminate the use of this snubber circuitry) has been developed, thereby making it possible to achieve enhanced performance. The GCT has a maximum cut-off current of 6,000A, and a turn-off accumulation time of less than or equal to 3 microseconds (.mu.s).
FIG. 6 is a cross-sectional view of a background compression bonded type semiconductor disclosed in Published Japanese Patent Application No. 8-330572 (1996), and which is designed to include a GCT and its associative gate drive device for controlling the GCT. As shown, a GCT 1 includes a semiconductor substrate 2. An aluminum gate electrode 2a is formed at an outer periphery on a top surface of the substrate 2, and a cathode electrode 2b is formed on an inner periphery of the top surface of the substrate. In addition, an anode electrode 2c is formed on a bottom surface of the substrate 2. A cathode distortion buffer disk 3 and an external cathode electrode 4 are sequentially stacked over each other on the side of the cathode electrode 2b. An anode distortion buffer disk 5 and an external anode electrode 6 are sequentially stacked on a side of the anode electrode 2c. In addition, the GCT 1 includes a ring gate electrode 7 made of molybdenum, which is in contact with the gate electrode 2a of the semiconductor substrate 2, and a ring-shaped external gate terminal 8 made of either iron or nickel alloy.
An inner periphery of the external gate terminal 8 contacts the ring gate electrode 7 and an outer periphery externally projects from a lateral side of an insulating cylinder 14. Further, curved portions 8a of the external gate terminal 8 are formed inside and outside of the insulating cylinder 14, and a specified number of attachment holes 8c (for example, twenty-four for a GCT of 6 kV/6 kA rating) are formed in connection portions 8b. The attachment holes 8c are for connecting the external gate terminal 8 to a plate-shaped control gate electrode 18 at equally spaced positions of a concentric pattern.
The GCT 1 also includes an elastic body 9, which presses the ring gate electrode 7 against the gate electrode 2a along with the external gate terminal 8 in cooperation with an annular insulator 10. Also provided are an insulator 11, a first flange 12 rigidly secured to the external cathode electrode 4 and a second flange 13 fixed to the external anode electrode 6. The insulating cylinder 14 is divided into upper and lower portions, and has an outer periphery that projects externally from a lateral side thereof and is rigidly attached by soldering at a divider section 14a. In addition, end portions 15 are soldered to the insulating cylinder 14 and then secured to the first flange 12 and second flange 13, thereby sealing the GCT 1.
In addition, a stack electrode 16 applies pressure to the GCT 1 and also takes out a current while simultaneously releasing heat from the external cathode electrode 4 and external anode electrode 6. A plate-shaped control electrode 17 includes an annular metal plate and is disposed concentrically with respect to the external gate terminal 8. A plate-shaped control gate electrode 18 includes an annular metal plate disposed concentrically with the external gate terminal 8 and is electrically connected to an outer periphery of the external gate terminal 8 at its inner periphery thereof. An insulation sleeve 19 electrically isolates the plate-shaped control electrode 17 and the plate-shaped control gate electrode 18, and is secured by fasteners 20. The plate-shaped control electrode 17 and plate-shaped control gate electrode 18 are connected with a gate drive device 21, which controls the GCT 1. A holding plate 23, such as a washer, functions as a distortion correction plate that firmly retains the connection portions 8b between the outer periphery of the external gate terminal 8 and the inner periphery of the plate-shaped control gate electrode 18 by use of fasteners 24 at each of the attachment holes 8c. Eighteen connection portions 8b may be provided for a 6 kV/4 kA-rated GCT (outer size is approximately 147 mm). Alternatively, twenty-four connection portions 8b may be used for a 6 kV/6 kA-rated GCT (outer size is about 200 mm).
An operation of the GCT 1 will now be explained. When the GCT 1 is turned on, a gate current is isotropically supplied from the gate drive device 21 to the external gate terminal 8 so the current is fed from the entire periphery thereof. Thus, a main current flows from the external anode electrode 6 toward the external cathode electrode 4. Alternatively, when the GCT 1 is turned off, a gate current of the reverse direction is supplied, thus rapidly extinguishing the main current. A current fall-down gradient of such a reverse gate current is set at approximately 6,000 A/.mu.s. This value setting makes it possible to increase the switching rate in cooperation with a rise-up gradient in the turn-on event at about 1,000 A/.mu.s.
However, the above-discussed background GCT 1 has the following problems.
As the maximum cutoff current increases, an increase in capacity of the GCT results in an increase in a number of segments that are concentrically parallel-connected on the surface of the semiconductor substrate 2. Thus further leads to an increase in a diameter of the semiconductor substrate 2 and a diameter of the package structure. In addition, the greater the outer diameter, the greater the number of attachment holes are required.
During product test/inspection procedures of the GCT 1, when the gate drive device 21 is limited in number, product test/inspection processes require repeated exchanges of the GCT 1. This requires time-consuming processes including complete attachment or detachment of the fasteners 24 to fix the attachment sections 8b. For example, in the product test procedure (turn-on test and turn-off test by pulse test/inspection techniques at high temperatures or low temperatures) of a GCT 1 of 6 kV/6 kA ratings, at least three processes of attachment and detachment of twenty-four different clamping parts is required. Even more complex processes and time consumption will be required with a further increased capacity of the GCT.
In addition, the holding plate 23 is designed to function as a distortion corrector plate to retain the contact between the outer periphery of the external gate terminal 8 and the inner periphery of plate-shaped control gate electrode 18. However, when the holding plate 23 has a relatively small thickness, the resulting pressure near or around a fixation portion of the fasteners 24 tends to become stronger. Thus, a close contact is achieved only at very limited portions adjacent to the fixation part in the connection portions 8b. This results in point-to-point or "pin-point" contact. Due to the lack of area contact, it is impossible to take full advantage of the GCT 1's inherent performance, such as an ability to supply a uniform gate current to the external gate terminal 8. This causes a serious problem in which the current locally concentrates which can permanently damage the GCT 1.
In addition, the increase of the switching speed or rate of the GCT 1 has widened the application field of large current controllability in certain operating frequency ranges exceeding 1 kHz, for example, especially where the external gate terminal 10 is made of specific ferromagnetic materials including iron or nickel. However, variations of magnetic fluxes induced by a recurrent phase inversion of a gate current can cause induction heat-up activities due to the electromagnetic induction, which results in an increase in temperature of the external gate terminal 8. Further, it is difficult to directly cool the external gate terminal 8 because of the component shape and layout, irrespective of the material of the external gate terminal 8.
The trend of further increasing the device capacity by increasing the maximal cut-off current of the GCT 1 also causes increases in the temperature of the gate electrode 2a. In contrast to the cathode electrode 2b and anode electrode 2c that are effectively cooled down, the cooling of the gate electrode 2a at the edge portion of the semiconductor substrate 2 is insufficient which results in along-the-surface temperature distribution of the semiconductor substrate 2 becoming non-uniform, causing the characteristics of the GCT 1 to change.